This invention relates to closed-throttle downshifting of an automatic transmission, and more particularly to transmission clutch control for synchronizing the transmission input speed with a target speed corresponding to the downshifted speed ratio.
In general, a motor vehicle automatic transmission includes a number of gear elements and selectively engageable friction elements (referred to herein as clutches) that are controlled to establish one of several forward speed ratios between the transmission input and output shafts. The input shaft is coupled to the vehicle engine through a fluid coupling such as a torque converter, and the output shaft is coupled to the vehicle drive wheels through a differential gearset. Shifting from a currently established speed ratio to new speed ratio involves, in most cases, disengaging a clutch (off-going clutch) associated with the current speed ratio and engaging a clutch (on-coming clutch) associated with the new speed ratio.
The present invention is directed to downshifts, and in particular, to downshifts performed during coast or braking conditions with the engine throttle essentially closed, such shifts being referred to herein as closed-throttle downshifts. Various clutch controls have been proposed for controlling closed-throttle downshifting. For example, the U.S. Patents to Lentz et al. 5,046,174 and Runde et al. 5,211,079 are directed to a control in which the off-going clutch is released, and the transmission input speed is raised to the synchronous speed of the target speed ratio by a closed-loop control of the on-coming clutch based on the deviation of the on-coming clutch slip from a desired slip profile. In another type of control, disclosed in the U.S. Patent to Downs et al. 4,671,139, the off-going clutch is released after the transmission input speed falls below the engine idle speed, and the closed-throttle engine torque raises the input speed to the synchronous speed of the target speed ratio, whereupon the on-coming clutch is engaged.
While the control disclosed by Downs et al. has the potential of achieving high quality closed-throttle downshifts with no negative output torque disturbance, clutch control variability makes it difficult to consistently achieve the desired timing for on-coming clutch engagement. This is particularly the case for shifts occurring at very low vehicle speeds, such as the shift to the first or lowest speed ratio. Accordingly, what is needed is a control that produces consistent high quality closed-throttle downshifts in spite of such variability.
The present invention is directed to an improved control for an automatic transmission closed-throttle downshift, wherein closed-throttle engine torque is used to raise the transmission input speed to the synchronous speed of the target speed ratio, and wherein the on-coming clutch is controlled in a manner to complete the shift with minimum driveline torque disturbance in spite of clutch control variability. The invention includes a primary on-coming clutch control for completing the downshift when the input speed reaches the synchronous speed, and a contingent on-coming clutch control that is initiated if shift completion is not achieved by the primary control, the contingent control being effective to re-establish input speed synchronization and thereupon engage the on-coming clutch to complete the shift. The primary clutch control involves holding the on-coming clutch in readiness for engagement while the input speed approaches the synchronous speed, and when the synchronous speed is reached, raising the on-coming pressure by a predetermined amount designed to maintain the input speed at the synchronous speed. The contingent clutch control includes a first open loop stage that progressively increases the on-coming clutch pressure to gradually re-establish the input speed synchronization, and a second closed-loop stage that adjusts on-coming clutch pressure to conform the input speed to a defined trajectory. Together, the primary and contingent controls ensure minimum disturbance shift completion in spite of clutch control variability.